Co-ordination method for a regenerative and anti-skid braking system

ABSTRACT

In a method, which is intended for a motor vehicle with a regenerative and an anti-lock conventional brake system (ABS) and is used for coordinating the application of the regenerative and the anti-lock system, the regenerative brake system is switched off upon entry into an ABS control, however, upon termination of the ABS control or ABS control phase, regenerative braking is admitted again in dependence on the respective driving situation and/or criteria representative of the brake demand and the instantaneous coefficient of friction, in a modified form as compared to the regenerative braking operation prior to the entry into the ABS control mode.

TECHNICAL FIELD

The present invention relates to a method for a motor vehicle with aregenerative and an anti-lock conventional brake system (ABS) forcoordinating the application of the regenerative and the anti-locksystem, wherein the regenerative brake system is switched off upon entryinto an ABS control.

BACKGROUND OF THE INVENTION

In vehicles with a regenerative brake (electric and hybrid vehicles), amajor part of the braking power or the total braking power is generatedby the electric drive when working on generator in the partial brakingrange, and the recuperated energy is fed back into the battery. Toachieve a high efficiency, the total braking power is generated by thegenerator, if possible, and thus applied only to the driven axle. On alow coefficient of friction, such as on ice, this may initiate ABScontrol because the coefficient of friction on the driven axle is notsufficient to transmit the required brake torque. When the ABS system isactive, the regenerative brake is switched off to uncouple the wheelsfrom the driving track and safeguard the normal ABS function.

In the following, the example is based on a vehicle with electric orelectro-hybrid drive (electric and combustion engine) and EHB(electro-hydraulic) brake.

The following problems are encountered in practical operations:

When the ABS control is terminated, it would be favorable to readmit theregenerative brake. However, this would shift the total braking power tothe driving axle again, and permanent cycles (cyclic change-overABS—regenerative brake—ABS—etc.) could occur. This condition is felt tobe unpleasant by the driver and has adverse effects on drivingstability.

DE 198 42 472 A1 discloses a vehicle brake system with friction devicesand regenerative brake devices. In this arrangement, the regenerativebrake is disconnected upon entry into ABS control. It stays disconnecteduntil the braking operation is completed. This condition is detected bythe fact that the driver accelerates or releases the brake duringstandstill.

BRIEF SUMMARY OF THE INVENTION

An object of the invention is to overcome the above-mentionedshortcomings and develop a method of the type referred to hereinabove,which permits improving the application of regenerative brake systems.

It has shown that this object can be achieved by means of the describedmethod of coordinating the application of a regenerative and ananti-lock system, the special features involving that regenerativebraking by means of the regenerative brake system upon termination ofthe ABS control or ABS control phase is re-admitted in dependence on therespective driving situation and/or criteria representative of thebraking demand and the instantaneous coefficient of friction, in amodified form compared to the regenerative braking operation prior toentry into the ABS control mode.

Thus, the invention involves a special method that permits coordinatingthe use of a regenerative brake and a friction brake in such a fashionthat after an ABS control operation the regenerative brake is stepwiseconnected again. Permanent cycles are prevented. The method purposefullygropes for the optimum of driving stability, comfort, and effective useof the regenerative brake (efficiency), and both constant and variablemarginal conditions represented by the coefficient of friction and thebraking demand are taken into consideration.

The sub-items 1 to 6 mentioned in the text correspond to conditions 1 to5 listed in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Upon termination of an ABS control phase, the regenerative brakingaccording to the method of the invention is admitted again in a modifiedform.

A distinction is made between the following conditions:

1. In the partial braking range (before the entry into the ABS control),the demanded braking power up to a maximum value predetermined by thegenerator is realized by means of the regenerative brake, any exceedingdemand is realized by means of the wheel brake.

2. The regenerative brake is deactivated upon entry into the ABScontrol. The demanded braking power is completely realized by means ofthe wheel brake (in the ABS control).

3. After the end of the ABS control, a timer is waited for to run down.The regenerative brake remains disconnected (like 2.) during this time(e.g. 2 seconds). The braking demand is realized only by way of thewheel brake. The pressure in the wheel brake cylinders at the end of theABS control is determined, converted into a brake torque and stored.

4. The regenerative brake is switched on again upon expiry of 3.However, the brake torque is limited. The limit is raised with a presetgradient starting from 0 until a defined portion (e.g. 80%) of the braketorque of the driving axle determined in 3. has been reached at the endof the ABS control. The wheel brake generates the braking power stillnecessary. In this condition, the brake force distribution onto theaxles is not changed in comparison to the end of the ABS control. A newABS entry is not possible with unmodified marginal conditions(coefficient of friction, braking demand).

5. Upon expiry of 4., the limit for the brake torque of the regenerativebrake continues to be slowly raised until the maximum valuepredetermined by the generator is reached. The wheel brake generates thebraking power exceeding the maximum value. In this condition, the brakeforce distribution to the axles is gradually shifted back to the drivingaxle. A new ABS entry is possible with unmodified marginal conditions(coefficient of friction, braking demand).

6. Upon expiry of 5., the regenerative brake is fully activated again.Thus, item 1 is resumed again. Item 6 corresponds to 1.

Should an entry into the ABS control take place in the conditions 3-5prior to transition into the next cycle, there will be a transition intocondition 2, and the cycle is continued from there. To avoid permanentcycles, the number of runs is limited as follows:

-   -   With a new ABS entry in state 5, a stop is made in 4. during the        next run (no more transition to 5.). The limit for the brake        torque of the regenerative brake hence remains on the value        reached in 4.    -   With a new ABS entry in state 4, a stop is made in 3. during the        next run (no more transition to 4.). The regenerative brake        stays deactivated.    -   With a new ABS entry in state 3, the number of runs is not        limited. There is no detrimental activation and deactivation of        the regenerative brake.        Hence, the following runs are possible after an ABS entry:

a) 1 - 2 - 3 - 4 - 5 - 1 . . .

-   -   The cycle was run through completely. A new ABS entry is        possible only by meanwhile modified marginal conditions (braking        demand of the driver, coefficient of friction). Permanent cycles        (multiple runs) are unlikely. Therefore, a new run is allowed.

b) 1 - 2 - 3 - 4 - 5 - 2 - 3 - 4

-   -   The coefficient of friction was not sufficient to completely        re-start the regenerative brake. Therefore, there is a new ABS        entry in 5. When the marginal conditions are unmodified, there        is no new entry into ABS control from state 4. The regenerative        brake stays activated with a reduced power.

c) 1 - 2 - 3 - 4 - 2 - 3

-   -   The coefficient of friction reduces in state 4, and a new ABS        entry takes place. Subsequently, the regenerative brake stays        deactivated in state 3.

d) 1 - 2 - 3 - 4 - 5 - 2 - 3 - 4 - 2 - 3

-   -   The coefficient of friction reduces after the run like b), and a        new ABS entry takes place. Subsequently, the regenerative brake        stays deactivated in state 3 like in c). Combination of b) and        c).

e) 1 - 2 - 3 - 2 - 3 - 2 . . .

-   -   Permanent cycle without detrimental effect. The regenerative        brake stays deactivated.

When the cycle is stopped as described hereinabove, a timer is started.Upon expiry of a long period of time (e.g. 30 seconds), or when thedriver accelerates, or no more brake torque is demanded, the cycle isreleased again, that means, is continued at the point where it washalted. The period is chosen to be so long that one can assume that thedriver no longer notices the correlation as disturbing. Besides,constant marginal conditions over a long period of time are veryunlikely.

Advantages:

During braking operations on an inhomogeneous coefficient of friction,the overall efficiency is greatly enhanced by the extended applicationof the regenerative brake.

Further embodiments are as follows:

-   -   1. Downhill driving over a long distance with a permanently        applied wheel brake. A patch with a low coefficient-of-friction        at the beginning of the course triggers ABS, thereafter follow        only high coefficients of friction. Such conditions are typical        on an Alpine pass. According to the prior art, the regenerative        brake would be deactivated and, first of all, the total energy        employed during uphill driving would be lost in charging of the        traction battery and, secondly, would considerably heat up the        wheel brake. According to the novel method, almost the total        energy can be used for charging, and the wheel brake is relieved        from load.

2. Road disturbance at the beginning of the braking operation,thereafter, high coefficients of friction. This is typical of highwayswith a poor roadway quality. Advantages like 1.

3. Braking on ice. The illustrated method prevents that several cyclesof regenerative brake—ABS follow each other. This avoids sequences ofbrake force shift (front axle—rear axle) that are unfavorable for thestability of the vehicle, and comfort is enhanced by a homogeneouscourse of the braking operation.

Further variants:

-   -   A multiple run of cycle a) can be prevented according to the        same principle.    -   The described gradients and times can be variable and e.g.        depend on the coefficient of friction.

1-9. (canceled)
 10. Method for a motor vehicle with a regenerative andan anti-lock conventional brake system (ABS) for coordinating theapplication of the regenerative and the anti-lock system, wherein theregenerative brake system is switched off upon entry into an ABScontrol, wherein regenerative braking by means of the regenerative brakesystem upon termination of the ABS control or ABS control phase isadmitted again in dependence on criteria representative of the brakingdemand and the instantaneous coefficient of friction, in a modified formcompared to the regenerative braking operation prior to the entry intothe ABS control mode.
 11. Method as claimed in claim 10, wherein priorto the entry into ABS control, the demanded braking power, i.e.corresponding to the driver's demand, is generated by the regenerativebrake system until the attainment of a maximum value depending on theregenerative brake system, while it is generated by the conventionalbrake system after entry into ABS control.
 12. Method as claimed inclaim 10, wherein the regenerative brake system is connected again aftertermination of an ABS control or an ABS control mode and after expiry ofa predetermined time period after the termination of the ABS control,and the brake torque generated by the regenerative brake system islimited to a predetermined limit value and the portion of the demandedbrake torque exceeding the limit value is generated by the conventionalbrake system.
 13. Method as claimed in claim 11, wherein thepredetermined time period is in the order of some seconds, e.g. 1 to 3seconds.
 14. Method as claimed in claim 12, wherein the predeterminedlimit value is varied in dependence on the locking pressure level, i.e.on the braking pressure that prevails in the wheel brake upontermination of the ABS control.
 15. Method as claimed in claim 12,wherein the predetermined limit value is raised with a preset gradientuntil an allowable maximum portion of the demanded braking power isreached and the portion of the demanded brake torque exceeding the limitvalue is generated by the conventional brake system.
 16. Method asclaimed in claim 15, wherein following a predetermined waiting timeafter the attainment of the allowable maximum portion of the demandedbraking power, a rise of the allowable maximum portion with apredetermined gradient is raised until the maximum output of theregenerative brake system is reached, and the portion of the demandedbrake torque exceeding the braking power of the regenerative brakesystem is generated by the conventional brake system.
 17. Method asclaimed in claim 10, wherein in a vehicle with only one driven axle, thebrake force distribution is shifted in favor of the driven axle when thedemanded braking power of the driven axle can be generated by theregenerative brake system.
 18. Method as claimed in claim 10, whereinupon a new entry into an ABS control after a transition into phases withregenerative braking, the new transition into phases of regenerativebraking is aggravated.
 19. Method as claimed in claim 10, wherein upon anew entry into an ABS control after a transition into phases withregenerative braking, the new transition into phases of regenerativebraking is delayed.